Boiling water nuclear reactors typically include a reactor core located within a reactor pressure vessel (RPV). A known RPV includes a substantially cylindrical shell. The shell, for example, can be about twenty feet in diameter and about seven inches thick. A substantially cylindrical core shroud is positioned within, and spaced from, the shell walls to restrain horizontal movement of the reactor core fuel bundles and provides flow separation between the downcomer and the upward core flow. Specifically, the shroud is a cylindrical, steel structure which surrounds the core and provides a barrier to separate the upward flow through the core from the downward flow in the annulus between the shroud and the reactor pressure vessel. The lower section of the shroud supports several components. For example, the core plate and two sets of in-core guide tube restraints are supported by the shroud lower section. The core plate also supports the top end of the in-core guide tubes.
Sections of the shroud are subjected to high fluence levels. For example, the upper section of the shroud, opposite the core, is in a high fluence area. The lower section of the shroud, below the core, is in a lower fluence area. Irradiation is known to assist in the cracking of some BWR materials. Thus, the upper shroud section may be more susceptible to irradiated assisted stress corrosion cracking (IASCC) than the lower section of the shroud because of high fluence levels.
Some known shrouds include a bolted joint at the bottom of the core. Typically, however, the shroud is formed by welding a plurality of stainless steel cylindrical sections together. Specifically, respective ends of adjacent shroud sections are joined with a circumferential weld. Thus, if the upper section of the shroud needs to be replaced, the entire shroud may have to be removed. Removing the entire shroud requires the removal of the core plate and other components. In some known reactor configurations, the top grid is bolted to the top of the shroud, and the top grid can be removed and replaced without requiring removal of the shroud.
It would be desirable to provide a shroud which allows the removal of an upper section of the shroud without requiring that the lower section of the shroud and the core plate need to be removed. It also would be desirable that such a shroud provide the same functionality as known shrouds.